HVAC systems are commonly configured to heat and cool air in an enclosed space, and may also constantly circulate the air to aid in maintaining a comfortable environment. HVAC systems may also be configured to introduce some quantity of outside air from time to time into the enclosed space as necessary to reduce or dilute various air contaminants such as carbon dioxide, or various vaporous or volatile organic compounds which may be irritating or potentially hazardous to the occupants. Introducing this outside air may require the exchange of previously conditioned air thus potentially increasing the thermal load on the HVAC system and making it less energy-efficient to operate. Furthermore, certain minimum air quality standards have been adopted by most building inspection authorities requiring that certain levels of air exchange the maintained, or that the level of certain air contaminants be monitored and regulated. This creates reduced efficiency as equipment and operating costs compliance requirements dictate increases in the rate of air exchange.